In general, in a single crystal manufacturing apparatus based on the pulling Czochralski method, the pressure inside a highly pressure proof airtight chamber is reduced to about 10 torr and fresh argon (Ar) gas is sent into it. Then, polycrystal in a quartz crucible placed in the lower portion of the chamber is heated and melted. Then, seed crystal is immersed into the surface of the melt from above, and the seed crystal is pulled while rotating and moving up and down the seed crystal and the quartz crucible. As a result, a single crystal (the so-called ingot) is grown, which comprises an upper cone portion in conical shape with its upper end protruding below the seed crystal, a body portion in cylindrical shape, and a lower cone portion in conical shape with its lower end protruding.
As the growing method, Dash method is known, in which a seed crystal is immersed into the surface of the melt to exclude dislocation (to turn to dislocation-free) which occurs to the seed crystal by thermal shock when the seed crystal is immersed in the surface of the melt. Then, pulling rate is relatively increased to form a neck portion with diameter smaller than the-diameter of the seed crystal, e.g. 3-4 mm, and pulling of the upper cone portion is started.
Because it is not possible to pull single crystal of larger diameter and heavier weight (150-200 kg or more) via the neck portion with small diameter, a method has been proposed, for example, in the Japanese Patent Publication 5-65477. According to this method, a neck portion with smaller diameter is formed by Dash method, and pulling rate is then relatively slowed down to form a portion with larger diameter. Then, the pulling rate is relatively increased to form a portion with smaller diameter, i.e. to form a "spherical constricted portion", and by gripping the constricted portion by grippers, single crystal of larger diameter and heavier weight is pulled up. Also, as a conventional apparatus for gripping the constricted portion, there are the apparatuses described in the Japanese Patent Publications 7-103000 and 7-515 in addition to the above patent publication.
As the other conventional examples, the following methods have been proposed: a method to grip a body portion without forming the "constricted portion" as shown in the Japanese Patent Publication Laid-Open 5-270974 or 7-172981, or a method to form "an annular constricted portion" with diameter larger than the body portion between the upper cone portion and the body portion and to grip this "annular constricted portion".
However, single crystal pulling process is a process, which is very sensitive to external vibration and which is very likely to cause polycrystal. Therefore, in single crystal pulling operation, there is a problem of how to move self-weight of the crystal in smooth and soft manner to gripping position such as constricted portion or the neck portion. Also, because of high temperature in the pulling furnace, gripping mechanisms and the like must be designed in heat-resistant construction. Further, intermingling of particles (dust) must be avoided as much as possible because this deteriorates quality of the crystal. None of the conventional techniques has definitely solved these problems. In the Japanese Patent Publications Laid-Open 5-270974, 5-270975, and 5-301793, the seed crystal pulling-mechanism and the gripper pulling mechanism have different driving sources, and there is neither disclosure nor suggestion as to how synchronous operation of these mechanisms can be attained.
In the Japanese Patent Publication Laid-Open 9-2893, the seed crystal pulling mechanism and the gripper pulling mechanism have different driving sources, and it is disclosed that these are synchronously operated or independently operated. However, in case of synchronous operation, the control process is complicated, and the apparatus used for control is complicated in structure and high in cost. Further, in the Japanese Patent Publication 7-515, it is disclosed that a vertical moving mechanism for engaging the grippers with constricted portion of single crystal is provided on a shaft to form the seed crystal pulling mechanism. After engagement, the grippers are also pulled up in synchronization with the seed crystal merely by the control of the seed crystal pulling mechanism. However, there is neither disclosure nor suggestion on effective means to stop the engaged portion, and it is difficult to use in practical application. Also, according to the Japanese Patent Publication 7-515 and the Japanese Patent Publication Laid-Open 7-172981, screw units and mechanical driving units in the driving mechanisms have metal components coming into contact with each other, and these are arranged in the pulling furnace maintained at high temperature. For this reason, it is difficult to take heat-resistant measures or mechanical units do not satisfactorily fulfill the function. Further, particles generated from mechanical contact parts may give adverse influence, and this hinders single crystal pulling operation.
According to the Japanese Patent Publication Laid-Open 9-2893, all driving mechanisms are arranged outside the vacuum chamber to avoid the problem of intermingling of particles into the pulling furnace under high temperature condition. By synchronous control of seed crystal pulling rate and gripping mechanism pulling rate, it is attempted to solve all problems found in the conventional type apparatus at once. However, the synchronous control can be achieved by mutually adjusting the pulling rate of wire drum to wind up seed crystal pulling wire and the pulling rate of the pulling drive, to which gripping means are connected. In this respect, the control mechanism and the control process are complicated and the cost for designing and manufacture of the apparatus is high, and it is also difficult to perform maintenance.